Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!husc6!cmcl2!beta!hc!ames!sdcsvax!darrell
From: Avadis.Tevanian@wb1.cs.cmu.edu (Avie)
Newsgroups: comp.os.research
Subject: Re: UNIX Facilities for Interpreters
Message-ID: <3378@sdcsvax.UCSD.EDU>
Date: Fri, 26-Jun-87 22:27:00 EDT
Article-I.D.: sdcsvax.3378
Posted: Fri Jun 26 22:27:00 1987
Date-Received: Sun, 28-Jun-87 01:34:59 EDT
Sender: darrell@sdcsvax.UCSD.EDU
Lines: 31
Approved: mod-os@sdcsvax.uucp

In article <3293@sdcsvax.UCSD.EDU|, boba@iscuva.ISCS.COM (Bob Alexander) writes:
| Modern, memory managed operating systems (like UNIX) have addressed
| quite nicely certain special requirements of executable files.  In
| particular (1) the file (text and data) need not be loaded into memory
| in its entirety to begin executing, and (2) the pages can be shared
| among processes that are executing them (both on disk and in memory).
| 
			:
| 
| For interpreters to take full advantage of this facility, they would
| have to interpret their p-code "as is" as it sits on disk.  If they
| modify the code, much of the advantage would be lost.
| 
| I'd be interested in hearing your comments and suggestions regarding
| this idea; alternative ideas to solve this problem, ways other OSs have
| dealt with it, implementation problems, or gross oversights.  What
| would you think of a "read only" option for this function (a fourth
| argument?), where the data would be mapped as read only (i.e.
| protected).

This type of stuff is trivial in modern operating systems like Mach (even
natural).  You simply map your file copy-on-write - all unmodified pages are
automatically demand paged and shared - modified pages are automicatally
copied as necessary.  Using a system like Mach, which maintains a virtual
memory cache of pages even after they are no longer in use, you can rerun a
program later (or access a file later) without any disk I/Os - tremendously
reducing total elapsed time of operations.  On machines with large memories
and/or high cpu speed to disk access time ratios performance wins can be
stunning.

	Avie